The obligation to name the source of one’s information:
Whoever does not name the source of his information transgresses a [Rabbinical] negative command. [Doing so is considered a form of stealing, and is even worse than the stealing of actual money. On the other hand, whoever says something in the name of its source brings redemption to the world. Nonetheless, the following cases of exception apply:]
- Source does not care to be named: One is not required to name the source of the information if one knows that the source does not care to be mentioned, although, nevertheless it is a Mitzvah to do so if the source can benefit from it.
- The information has become one’s personal knowledge: Likewise, one is not required to mention the source if he has internalized the information to the point that it has become his personal opinion. Thus, if one did his own independent research to corroborate it, or understand it in his mind, and has now acquired this idea as his own opinion, the original source who led him there does not need to be mentioned. Nonetheless, even in such a case, it is a Mitzvah to mention the source and bring redemption to the world [if he heard it from the person directly].
- Is potentially shameful: One is not to name the source of his information in the event that the person would be ashamed or does not want his name mentioned. Thus, for example, if the person is mentioning the information for the sake of arguing on it, then it is better to not quote the source and so is customary.]
If one is quoting something he read in a Sefer, based on prior sources, must all sources be mentioned?
Some Poskim write one is required to name up to three sources back. Other Poskim write that one is required to mention the original source and the last source that one read it in, however, not any other sources who quote from the original source. Others, however, write that one is not required to mention any other source than the original source, although its proper to also mention the Sefer one saw it in.
 M”A 156:1 [omitted in Admur 156-See Likkutei Sichos 36:180]; Shlah 73a; Sefer Chassidim 586; Nedarim “Tani Rebbe Chiya..” [so writes M”A and Shlah as its source. This is not found amongst our prints-See Likkutei Sichos 36:180] Tanchuma Bamidbar 22 “Rebbe Chizkiya stated in name of Rebbe Yochanon whoever does not..”; Yalkut Shimoni Bamidbar 695 and Mishlei 960; Sifri; Shaareiy Teshuvah 156:2; Piskeiy Teshuvos 156:27; Likkutei Sichos 36:180 [printed in Shulchan Menachem 1:324]; See also Yevamos 96b regarding story with Rebbe Elazar; Michaber Y.D. 242:24; Shach Y.D. 242:43 “Certainly it is forbidden to say Stam, and all the more in one’s own name, something which he heard from another, as he is wearing a Tallis that is not his.”
 See Tanchuma ibid
 The reason: As the verse [Mishlei 22:22] states “Do not steel from the poor.” [Shlah ibid in name of Nedarim ibid; Tanchuma ibid] Alternatively, as the verse [Devarim 19:14] states “Do not steal your friends boundary.” [Sefer Chassidim ibid]
 Implication of Tanchuma ibid; Shlah 183b; See Likkutei Sichos 36:180 in length
 Shlah 183b, brought in Likkutei Sichos 36:180
 Avos 6:6; Megillah 15a; Chulin 104b; Niddah 19b; See Likkutei Levi Yitzchak on Zohar 2:140, brought in Likkutei Sichos 36:180, that this only applies if one heard it from the person directly, as was the case with Mordechai and Esther
The reason: This is learned from the verse [Esther 2:22] which states “And Esther told the king in the name of Mordechai.” [Megillah ibid] By Esther telling Achashveirosh of the plot in the name of Mordechai she helped bring along the Purim miracle.
 See Likkutei Sichos 36:180
 See Likkutei Sichos 36:180
 Likkutei Levi Yitzchak on Zohar 2:140, brought in Likkutei Sichos 36:180
 Piskeiy Teshuvos ibid; See Toras Menachem 5743 4:1837 [printed in Shulchan Menachem 1:324]
 Toras Menachem 5743 4:1837 [printed in Shulchan Menachem 1:324]
The reason: It is better to forgo the reward of naming one’s source in such a case in order to negate any chance of embarrassing the source of the information or feeling pride on the expense of his negation. [ibid]
 M”A in Zayis Ranaan on Yalkut Shimoni ibid, brought in Likkutei Sichos 36:180
 Gloss of Chochmas Shlomo on 156
 See Piskeiy Teshuvos 156:27 footnote 244