Musnad Ahmad 25:340

Ammar ibn Muhammad, the nephew of Sufyan al-Thawri, narrated to us, from Mansur, from Salim, from Kurayb, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Is one of you unable, when he comes to his wife, to say: 'In the name of Allah, O Allah, keep Satan away from me, and keep Satan away from what You provide for me'? For if Allah decrees a child between them from that, Satan will never harm him."