Understanding the nature and origin of the precursor material to terrestrial planets is key to deciphering the mechanisms and timescales of planet formation¹. Nucleosynthetic variability among rocky Solar System bodies can trace the composition of planetary building blocks^2–5. Here we report the nucleosynthetic composition of silicon (μ³⁰Si), the most abundant refractory planet-building element, in primitive and differentiated meteorites to identify terrestrial planet precursors. Inner Solar System differentiated bodies, including Mars, record μ³⁰Si deficits of −11.0 ± 3.2 parts per million to −5.8 ± 3.0 parts per million whereas non-carbonaceous and carbonaceous chondrites show μ³⁰Si excesses from 7.4 ± 4.3 parts per million to 32.8 ± 2.0 parts per million relative to Earth. This establishes that chondritic bodies are not planetary building blocks. Rather, material akin to early-formed differentiated asteroids must represent a major planetary constituent. The μ³⁰Si values of asteroidal bodies correlate with their accretion ages, reflecting progressive admixing of a μ³⁰Si-rich outer Solar System material to an initially μ³⁰Si-poor inner disk. Mars’ formation before chondrite parent bodies is necessary to avoid incorporation of μ³⁰Si-rich material. In contrast, Earth’s μ³⁰Si composition necessitates admixing of 26 ± 9 per cent of μ³⁰Si-rich outer Solar System material to its precursors. The μ³⁰Si compositions of Mars and proto-Earth are consistent with their rapid formation by collisional growth and pebble accretion less than three million years after Solar System formation. Finally, Earth’s nucleosynthetic composition for s-process sensitive (molybdenum and zirconium) and siderophile (nickel) tracers are consistent with pebble accretion when volatility-driven processes during accretion and the Moon-forming impact are carefully evaluated.