Sayyiduna Abu Sa'id (may Allah be pleased with him) said: We asked the Messenger of Allah (peace and blessings be upon him) about 'azl, and he (peace and blessings be upon him) said: "It is not required of you to do this, for it is only the decree of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14310