Answer is: sp3 on nitrogen overlaps with 1s on hydrogen.
Electron
configuration for nitrogen: ₇N
1s² 2s² 2p³.
Electron
configuration for hydrogen: ₁H 1s¹.
The central atom in ammonia, nitrogen, is sp3 hybridized (one s and three p-orbitals will combine for a four sp3 hybridized orbitals), because sp3 orbitals have lower energy.