It is legal if somebody hires someone to work for him after three days, or after one month or after a year. When that period elapses they should carry out their contract

Sahih al-Bukhari · Book 37 · Hadith 2264

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، قَالَ ابْنُ شِهَابٍ فَأَخْبَرَنِي عُرْوَةُ بْنُ الزُّبَيْرِ، أَنَّ عَائِشَةَ ـ رضى الله عنها ـ زَوْجَ النَّبِيِّ صلى الله عليه وسلم قَالَتْ وَاسْتَأْجَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَأَبُو بَكْرٍ رَجُلاً مِنْ بَنِي الدِّيلِ، هَادِيًا خِرِّيتًا وَهْوَ عَلَى دِينِ كُفَّارِ قُرَيْشٍ،…

Narrated Aisha: (the wife of the Prophet) Allah's Messenger (ﷺ) and Abu Bakr hired a man from the tribe of Bani-Ad-Dil as an expert guide who was a pagan (follower of the religion of the pagans of Quraish). The Prophet…