Sahih Bukhari Volume 3, Book 50, Hadith Number 874.
“O you who believe! When the believing women come to you as emigrants. Examine them, Allah knows best as to their belief, then if you know them for true believers, Send them not back to the unbelievers, (for) they are not lawful (wives) for the disbelievers, Nor are the unbelievers lawful (husbands) for them (60.10).
Narrated ‘Urwa: ‘Aisha told me, “Allah’s Apostle used to examine them according to this Verse: “O you who believe! When the believing women come to you, as emigrants test them… for Allah is Oft-Forgiving, Most Merciful.” (60.10-12) ‘Aisha said, “When any of them agreed to that condition Allah’s Apostle would say to her, ‘I have accepted your pledge of allegiance.’ He would only say that, but, by Allah he never touched the hand of any women (i.e. never shook hands with them) while taking the pledge of allegiance and he never took their pledge of allegiance except by his words (only).”