Hi Hugo, welcome, and good question. I’m hoping someone comes up with an answer for you; nothing immediately springs to my mind. But if we don’t get an answer in a day or two I’ll see what I can find.
I found this sutta thag 15.1 but it’s not spoken by the Buddha, but rather Koṇḍañña. It’s probably not the one you’re looking for but it does talk about being humble and stable and with no need for students. But it’s about awakening, not specifically about remaining in robes.
Also found sn16.8, spoken by the Buddha but also probably not the one you’re looking for.
And here’s one of my favorite suttas, though again probably not be the one you’re thinking of:an8.30
“And what, bhikkhus, are the eight thoughts of a great person? (1) This Dhamma is for one with few desires, not for one with strong desires. (2) This Dhamma is for one who is content, not for one who is discontent. (3) This Dhamma is for one who resorts to solitude, not for one who delights in company. (4) This Dhamma is for one who is energetic, not for one who is lazy. (5) This Dhamma is for one with mindfulness established, not for one who is muddle-minded. (6) This Dhamma is for one who is concentrated, not for one who is unconcentrated. (7) This Dhamma is for one who is wise, not for one who is unwise. (8) This Dhamma is for one who delights in non-proliferation, who takes delight in non-proliferation, not for one who delights in proliferation, who takes delight in proliferation.
I spent a lot of time yesterday looking in my browser history but couldn’t find it. I’m starting to think that I dreamed with the Buddha telling me those things