Khalid bin Abi Malik (may Allah have mercy on him) said: I sold something to Muhammad bin Sa'd (may Allah have mercy on him), and he said: Give me your hand so that I may clasp hands with you, for the Messenger of Allah (peace and blessings be upon him) said: "Blessing is in clasping hands."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11189