Musannaf Ibn Abi Shayba 13:2693

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Ibn Jurayj, from ‘Ata’, who said: There is no harm in paying in advance (salam) for what is measured with what is measured, and for what is weighed with what is weighed. It is only like food for food.