SuttaCentral
I remember a sutta where someone (possibly Sariputta?) is criticized or attacked, and other monks simply stand by. And then the Buddha says to the monks that they should have come to their brother’s aid. I’m not sure if the Buddha used the word “equanimity,” but I suspect he did.
I’ve been looking for this sutta for some time. Hopefully someone can help me locate it.
Thanking you in advance.
Based on your description, I give you my best shot (AN5.166) :
https://suttacentral.net/an5.166/en/sujato
However, I am not too sure this is the one you are looking for. 
That is the very one. I’d not remembered it very clearly, apparently. Thank you so much.
There is also a similar comment in MN 67. Here the Buddha describes the dangers of the ocean, which means crossing from the nearer to the further shore. On a raft in that situation equanimity would be appropriate on few occasions.
That’s great to hear 
I read the translation from Bhante @sujato but I don’t really understand this part, maybe someone can help here? 
Atha kho bhagavā āyasmantaṁ udāyiṁ āmantesi:
“kaṁ pana tvaṁ, udāyi, manomayaṁ kāyaṁ paccesī”ti?
“Ye te, bhante, devā arūpino saññāmayā”ti.
“Kiṁ nu kho tuyhaṁ, udāyi, bālassa abyattassa bhaṇitena.
Tvampi nāma bhaṇitabbaṁ maññasī”ti.
I don’t really see the similarity you’re talking about, although the Buddha does tell Sariputta off for thinking that he can just quietly get on with his own practice rather than exercising compassion by leading the sangha. This is one of my favorite suttas, by the way!
MN 101 states equanimity and right effort are both appropriate in different situations, but doesn’t say what those are. Remembering equanimity has an agenda, in my view this sutta points out right effort is the more used response in crossing to the further shore:
"The Buddha said to Venerable Sāriputta, “Sāriputta, what did you think when the mendicant Saṅgha was dismissed by me?”
“Sir, I thought this: ‘The Buddha has dismissed the mendicant Saṅgha. Now he will remain passive, dwelling in blissful meditation in the present life, and so will we.’”
“Hold on, Sāriputta, hold on! Don’t you ever think such a thing again!”
[…]
Then the Buddha said to the mendicants:
“Mendicants, when you go into the water you should anticipate four dangers. What four? The dangers of waves, crocodiles, whirlpools, and sharks. These are the four dangers that anyone who enters the water should anticipate."—MN 67 SC
You have search the source I don’t have time. Only found the qoute in my notes
Equanimity is of two sorts, I tell you: to be pursued & not to be pursued.’ Thus was it said. And in reference to what was it said? When one knows of a feeling of equanimity, ‘As I pursue this equanimity, unskillful mental qualities increase, and skillful mental qualities decline,’ that sort of equanimity is not to be pursued. When one knows of a feeling of equanimity, ‘As I pursue this equanimity, unskillful mental qualities decline, and skillful mental qualities increase,’ that sort of equanimity is to be pursued. And this sort of equanimity may be accompanied by directed thought & evaluation or free of directed thought & evaluation. Of the two, the latter is the more refined. ‘Equanimity is of two sorts, I tell you: to be pursued & not to be pursued.’
I did find the sutta I was looking for, but thanks for the reference. It’s DN 21, if you want to add the source to your notes.