Musnad Abu Ya'la al-Mawsili 160:215

Daud ibn 'Amr narrated to us, Habban ibn 'Ali al-'Anazi narrated to us, from Muhammad ibn 'Ajlan, from Sa'id al-Maqburi, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you buys a servant, let him take hold of her forelock and say: 'O Allah, I ask You for her good and the good of that which You have created her upon.' And if he buys a camel, let him take hold of the peak of its hump and say the like of that."