What is the ruling on saying "Indeed, Allah does not break the promise" after making a supplication?

It appears the question is about the addition of (إنك لا تخلف الميعاد) "Verily, You do not break Your promise" in the supplication after the Athan. This is an anomalous addition not found in the Sahihayn (Bukhari and Muslim). It was narrated by Al-Bayhaqi from Muhammad ibn Awf, while many leading memorizers of Hadith narrated it from Ali ibn Ayyash without this addition. It is preferable to stick to what is authentically reported in the Sahihayn: "Allahumma Rabba hadhihi-d-dawati-t-tammah, was-salati-l-qa'imah, ati Muhammadan al-wasilata wal-fadilah, wabathhu maqaman mahmoodan alladhi waadtah" (O Allah, Lord of this perfect call and established prayer, grant Muhammad the Wasilah and Al-Fadilah, and raise him to the praised station which You promised him). Some scholars, like Ibn Baz, have deemed this addition good. So, there is no harm for whoever acts upon it by way of تقليد (following a scholarly opinion) or sees its goodness. As for praising Allah with this phrase or seeking intercession through it, there is no harm in that.