Sunan Ibn Majah
2886
Sahih+3

It was narrated from Ibn ‘Abbas that Aqra’ bin Habis asked the Prophet (ﷺ):“O Messenger of Allah, is Hajj (required) every year, or just once” He said: “Rather it is just once. And whoever can perform Hajj voluntarily, let him do so.”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi