Sayyidah Aishah (may Allah be pleased with her) said that when the women came to him (peace and blessings be upon him) having emigrated, he (peace and blessings be upon him) would take their pledge of allegiance on verse 12 of Surah al-Mumtahanah: "O Prophet, when the believing women come to you pledging allegiance to you" [Surah al-Mumtahanah: 12]. When they affirmed it, he would say: "You have indeed given your pledge." And the Prophet (peace and blessings be upon him) never touched the hand of any unrelated woman.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16567