Sahih Muslim Book 8, Hadith Number 3394.

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

Sa’d b. Abu Waqqas (Allah be pleased with him) reported that a person came to Allah’s Messenger (May peace be upon him) and said: I do ‘azi with my wife. Thereupon Allah’s Messenger (May peace be upon him) said: Why do you do that? The person said: I fear harm to her child or her children. Thereupon Allah’s Messenger (way peace be upon him) said: If that were harmful it would harm the Persians and the Greeks.