Shulchan Arukh Orach Chayim · Siman 557

English

On Tisha Be-Av we say [during] the blessing of "Bo-neh Yerushalayim" {in the Amidah}: {Nachem} "Console My L-rd my G-d the mourners of Zion etc ..", and "Aneinu" during "Shomeya Tefilah", but if one did not say one nor the other, we do not make them go back. Rema Note: The simple custom is not to say Nachem except during the Minchah prayer on Tisha be-Av, for it was then {in the afternoon} that the fire took hold in the Mikdash, and therefore we pray at this time about the consolation (Rohach, Avudrahum). One who ate on Tisha be-Av, would say "Nachem" {instead of "Rachem"} in Grace after meals (Maharil).