An advanced understanding of ultrafast coherent electron dynamics is necessary for the application of submicrometre devices under a non-equilibrium drive to quantum technology, including on-demand single-electron sources(1), electron quantum optics(2-4), qubit control(5-7), quantum sensing(8,9) and quantum metrology(10). Although electron dynamics along an extended channel has been studied extensively(2-4,11), it is hard to capture the electron motion inside submicrometre devices. The frequency of the internal, coherent dynamics is typically higher than 100 GHz, beyond the state-of-the-art experimental bandwidth of less than 10 GHz (refs. (6,12,13)). Although the dynamics can be detected by means of a surface-acoustic-wave quantum dot(14), this method does not allow for a time-resolved detection. Here we theoretically and experimentally demonstrate how we can observe the internal dynamics in a silicon single-electron source that comprises a dynamic quantum dot in an effective time-resolved fashion with picosecond resolution using a resonant level as a detector. The experimental observations and the simulations with realistic parameters show that a non-adiabatically excited electron wave packet(15) spatially oscillates quantum coherently at similar to 250 GHz inside the source at 4.2 K. The developed technique may, in future, enable the detection of fast dynamics in cavities, the control of non-adiabatic excitations(15) or a single-electron source that emits engineered wave packets(16). With such achievements, high-fidelity initialization of flying qubits(5), high-resolution and high-speed electromagnetic-field sensing(8) and high-accuracy current sources(17) may become possible.