Al-Kāfi - Volume 5 > Working for Hire as Payment for Mahr
Hadith #2

2- عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ النَّوْفَلِيِّ عَنِ السَّكُونِيِّ عَنْ أَبِي عَبْدِ اللهِ (عَلَيْهِ السَّلام) قَالَ لا يَحِلُّ النِّكَاحُ الْيَوْمَ فِي الإسْلامِ بِإِجَارَةٍ أَنْ يَقُولَ أَعْمَلُ عِنْدَكَ كَذَا وَكَذَا سَنَةً عَلَى أَنْ تُزَوِّجَنِي ابْنَتَكَ أَوْ أُخْتَكَ قَالَ حَرَامٌ لأنَّهُ ثَمَنُ رَقَبَتِهَا وَهِيَ أَحَقُّ بِمَهْرِهَا.

2. Ali ibn Ibrahim has narrated from his father from al-Nawfaliy from al-Sakuniy who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘Marriage is not lawful in Islam for hiring and saying, ‘I will work for you up to so and so many years and you give to me in marriage your daughter or sister.’ He (the Imam) said, ‘It is unlawful because it is the price of her neck and in fact she is the rightful person for her mahr (dower).”’