Sahih Muslim Book 8, Hadith Number 3392.

Chapter: Permissibility of intercourse with a suckling lady and disapproval of ‘Azl.

Judama daughter of Wahb, sister of Ukkasha (Allah be pleased with her) reported: I went to Allah’s Messenger (May peace be upon him) along with some persons and he was saying: I intended to prohibit cohabitation with the suckling women, but I considered the Greeks and Persians, and saw that they suckle their children and this thing (cohabitation) does not do any harm to them (to the suckling women). Then they asked him about ‘azl, whereupon he said. That is the secret (way of) burying alive, and Ubaidullah has made this addition in the hadith transmitted by al-Muqri and that is: “When the one buried alive is asked.”