Mahavagga, Khandaka 1, Chapter 55

Then the Blessed One, after having resided at Kapilavatthu as long as he thought fit, went forth to Sāvatthi. Wandering from place to place he came to Sāvatthi. There the Blessed One dwelt at Sāvatthi, in the Jetavana, the Ārāma of Anāthapiṇḍika.

At that time a family who devoted themselves to the (especial) service of the venerable Sāriputta sent a boy to the venerable Sāriputta (with this message): 'Might the Thera confer the pabbajjā ordination on this boy.' Now the venerable Sāriputta thought: 'The Blessed One has established the rule^[1] that no one may ordain two novices, and I have already one novice, Rāhula. Now what am I to do?' He told the thing to the Blessed One.

“I allow, O Bhikkhus, a learned, competent Bhikkhu to ordain two novices, or to ordain as many novices as he is able to administer exhortation and instruction to.”