Riyad as-Salihin

'Abdullah bin 'Umar (May Allah be pleased with them) reported: Messenger of Allah (ﷺ) said, "It is the duty of a Muslim who has something which is to be given as a bequest not to have it for two nights without executing a written will."

[Al-Bukhari and Muslim].

In the narration of Muslim it is narrated as 'three nights'. Ibn 'Umar (May Allah be pleased with them) said: Since I heard the Messenger of Allah (ﷺ) say this, I have never spent a night without having my will with me.