On page 130 of From Heart and Hand, Vol. 2 by Ajahn Jayasaro, we find:
What sutta is this teaching found in? Thank you.
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Oh, that’s one of my favorite things!
As far as I know it’s in the Vinaya, not suttas. I’m having trouble finding it by searching.
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Ah, here it is:
kd19:5.7.5
“And how many qualities should a monk who is accused set up?”
“Cuditena pana, bhante, bhikkhunā katisu dhammesu patiṭṭhātabban”ti?“A monk who is accused should set up two qualities:
“Cuditenupāli, bhikkhunā dvīsu dhammesu patiṭṭhātabbaṁ—
truth and composure.”
sacce ca akuppe cā”ti.
ETA: The Vinaya teaching is similar to what it quoted in the OP, however the sutta citation given by folks below is exactly what the OP asks for.
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I think you are looking for [SuttaCentral] AN 5.167
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Edit: Sarnath’d by @Alex70 ! 
I think it’s also AN 5.167:
A person who is accused should ground themselves in two things:
Cuditena, āvuso, puggalena dvīsu dhammesu patiṭṭhātabbaṁ—
truth and an even temper.
sacce ca, akuppe ca.
Even if others accuse me—at the right time or the wrong time, truthfully or falsely, gently or harshly, beneficially or harmfully, lovingly or with secret hate—I will still ground myself in two things:
Mañcepi, āvuso, pare codeyyuṁ kālena vā akālena vā bhūtena vā abhūtena vā saṇhena vā pharusena vā atthasaṁhitena vā anatthasaṁhitena vā mettacittā vā dosantarā vā, ahampi dvīsuyeva dhammesu patiṭṭhaheyyaṁ—
truth and an even temper.
sacce ca, akuppe ca.
If I know that
Sace jāneyyaṁ:
that quality is found in me, I will tell them that it is.
‘attheso mayi dhammo’ti, ‘atthī’ti naṁ vadeyyaṁ:
‘saṁvijjateso mayi dhammo’ti.
If I know that
Sace jāneyyaṁ:
that quality is not found in me, I will tell them that it is not.”
‘nattheso mayi dhammo’ti, ‘natthī’ti naṁ vadeyyaṁ:
‘neso dhammo mayi saṁvijjatī’”ti.
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Only by a second @Dogen 
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Thank you, all!!!
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