Ubayy ibn ka'b (RA) said: "I said: 'O Messenger of Allah! Indeed I say very much Salat for you. How much of my Salat should I make for you?' He (ﷺ) said: 'As you wish.'" [He said:] "I said: 'A fourth?' He (ﷺ) said: 'As you wish. But if you add more it would be better for you.' I said: 'Then half?' He (ﷺ) said: 'As you wish. And if you add more it would be better [for you].'" [He said:] "I said: 'Then two-thirds? 'He (ﷺ) said: 'As you wish, but if you add more it would be better for you.' I said: 'Should I make all of my Salat for you?' He (ﷺ) said: 'Then your problems would be solved and your sins would be forgiven.' This hadith tell us to increase sending prayers upon the Prophet. Only authentically proved prayers from the Quran and Sunnah should be recited in this regard.

Reference: صحیح (آلبانی). ترمذی: ۲۴۵۷