The shahada is "the Muslim profession of faith: ‘There is no god but God, and Muhammad is the Messenger of God’."1
The most prominent shahada at Humayun's Tomb is at the top of the dome's finial. Robert Hillenbrand has this insight on the location of shahada inscriptions in locations so high.
Sometimes the religious inscriptions are so high that one might think they could be aimed only at God [...] invisible to the human eye. Familiarity with the sacred text on the part of those who frequented religious buildings could fairly be assumed, and this in turn meant that the recognition of a single word could unlock an entire inscription band.2
At Humayun's Tomb the shahada can be recognized in other places if one looks carefully. It appears in the designs on a few incised plaster medallions in one of the tomb's ancillary chambers. It can also be found on a few cenotaphs in the mausoleum and around Humayun's Garden Tomb.
1. Kishwar Rizvi, "Mosques and Commemorative Shrines: Piety, Patronage and Performativity in Religious Architecture," in Architecture in Islamic Arts: Treasures of the Aga Khan Museum, ed. Margaret S. Graves and Benoît Junod (Geneva: Aga Khan Trust for Culture, 2011), 77.
2. Robert Hillenbrand, "Splendour and Austerity: Islamic Architectural Ornament," Annual Halt, no. 2 (1995): 10.
Abduazizovich, Rahimov Laziz. "The Mausoleum Of Humayun." International Journal of Scientific & Technology Research 5, (July 2016): 209-14.
Hillenbrand, Robert. "Splendour and Austerity: Islamic Architectural Ornament." Halt Annual, no. 2 (1995): 8-27.