بسم الله الرحمن الرحيم
While mentioning the Prophet’s صلى الله عليه و سلام continual observance of the sunnan prayers Al Imam Ibnul Qayyim رحمه الله mentions that they total to 40 raka’at daily which he never abandoned or left. Even while sick.
He goes onto mention, “So it is befitting for the slave to be consistent upon these prayers until death, for how swift is the response and how quick is the opening of the door for the one who knocks on it 40times day and night?..”
[Mukhtasar Za’d ul Ma’ad; pg. 55]