Mustadrak al-Hakim — Hadith 2792

Amr bin Shu'ayb narrates from his father, and he from his grandfather (Sayyiduna Abdullah bin Amr bin al-As, may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) said: "When one of you acquires a slave girl, a wife or a riding animal, let him take hold of her forelock and pray for blessing, and let him say: 'Allahumma inni as'aluka khayraha wa khayra ma jubilat 'alayh, wa a'udhu bika min sharriha wa sharri ma jubilat 'alayh' (O Allah, I ask You for her good and the good of the nature upon which You created her, and I seek refuge in You from her evil and the evil of the nature upon which You created her). And if it is a camel, let him take hold of the top of its hump."
This hadith is sahih (authentic) according to the narrations that trustworthy imams have transmitted from Amr bin Shu'ayb, but the two Shaykhs did not record it in their books by way of Amr (bin Shu'ayb).