According to rahyafte (the missionaries and converts website): The merciful prophet of Islam, peace be upon him and his household says:
أعطُوا الأجيرَ أجرَهُ قَبلَ أنْ يَجِفَّ عَرَقُهُ، وأعْلِمْهُ أجرَهُ وهُو في عملِهِ.
‘Give the worker his wage before his sweat dries up, and inform him of his wage while he is still working.’
Source: Kanz al-Ummal, no. 9126
