Can we recite the following in Tashahhud: ʾashhadu ʾan lā ilāha ʾillā -llāhu...wa-rasūluhū.Bil haqqe bashiraw wa naziraw baina yadayas saaw ʾallāhumma ṣalli ʿalā muḥammadin wa-ʾāli muḥammadin?
Yes. It is narrated as one of the Mustahab (recommended) recitations in Tashahhud.
أشهد ان لا اله الا الله وحده لا شريك له واشهد ان محمدا عبده ورسوله ارسله بشيرا ونذيرا بين يدي الساعة . اللهم صل على محمد وآل محمد
It is mentioned in many jurisprudence books like al-'Orwatul Wothqa and other books beside our books of Hadeeth like Wasaa'il al-Shia.
Wassalam.