Maaser Sheni, Chapter 2, Mishnah 7:
"Beis Shammai omrim: lo ya'aseh adam es kaspo dinrei zahav" - according to Beis Shammai, a person may not take the silver coins in his possession and hold on to them until he has accumulated a sum large enough to exchange for gold coins. It takes a considerable number of silver coins to equal a single gold dinar, and Beis Shammai did not want a person to conduct himself this way.
The concern underlying the view of Beis Shammai:
If a person is given the option of converting his money into gold, he is liable to postpone his trip to Jerusalem until he has gathered enough silver coins to equal a gold dinar, and as a result he will miss the obligation of the pilgrimage on Pesach, Shavuos and Sukkos. For this reason they ruled that this option should not be available to him at all, lest he say: "I will wait until I have the value of a gold dinar in hand."
It is worth noting that carrying a few gold coins is far more convenient than carrying a bundle of silver coins.
The view of Beis Hillel:
Beis Hillel permit converting the silver coins into gold dinars. Even though accumulating the sum may take time, we are not concerned that a person will miss the pilgrimage because of it.
The testimony of Rabbi Akiva:
"Amar Rabbi Akiva: ani asisi l'Rabban Gamliel ul'Rabbi Yehoshua es kaspan dinrei zahav" - Rabbi Akiva testifies from actual practice in accordance with the view of Beis Hillel: he himself took the silver coins of Rabban Gamliel and Rabbi Yehoshua and combined them until they equaled a smaller number of gold dinars, and there was no concern that they would postpone the pilgrimage because of this. This testimony comes to support the ruling like Beis Hillel.
In summary: in this mishnah Beis Shammai and Beis Hillel disagree about converting silver coins into gold dinars: Beis Shammai forbid it, out of concern that a person will wait for the sum to accumulate and postpone the pilgrimage; Beis Hillel permit it and are not concerned about this. Rabbi Akiva testifies from an incident in which he did this for Rabban Gamliel and Rabbi Yehoshua, and this strengthens the ruling like Beis Hillel.