Is it sufficient to make the intention (niyyah) for prayer only in the heart?

Intention (niyyah) is one of the conditions of prayer. Niyyah is an act of the heart; it means that a person decides on something and knows for what purpose they are performing an act. In prayer, what is valid is the intention in the heart. Although it is recommended (mustahabb) to express the intention verbally, the prayer is still valid if it is not uttered. (Merğînânî, al-Hidāyah, 1/46) When performing obligatory (fard) or necessary (wājib) prayers, a person must specify which prayer they are performing. For sunnah prayers, however, it is not required to specify which time’s sunnah prayer is being performed. (Shurunbulālī, Marāqī al-Falāh, 85)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu