Tafsir of Ad-Dhariyat 51:30

They said: "Even so, your Lord has said." It means: such is the noble statement about which we have informed you. We are merely messengers conveying it to you on His behalf, the Exalted and Majestic, not that we are saying it of our own accord. It has been narrated that Gabriel, peace be upon him, said to her: "Look at the ceiling of your house." She looked, and behold, its rafters were leafy and bearing fruit. "Indeed, it is He who is the All-Wise, the All-Knowing."

Thus, His statement, may He be exalted, and His action, may He be glorified, will inevitably come to pass. This dialogue was not with Sarah alone, but it was also with Abraham, as has preceded in Surah al-Hijr. The reason it was not mentioned here is to suffice with what was mentioned there, just as it was not mentioned there to suffice with what was mentioned here and in Surah Hud.